Hello there and welcome to Just Answer. My name is ***** ***** I will be assisting you today. I am an engineer with over 30 years of electrical and electronic training, repair and installation experience. I will try and answer your question accurately and precisely so that we can get you on your way.
Ok, what you are doing is correct so far except you are missing one important component - you need to run the Garmin Express software on your computer. So...download the program from Garmin by clicking HERE.
Install it and then run it. When prompted, plug in your Nuvi like you have been doing and the program will take over and show you your update options.
Please let me know if you run into problems. I'll be here.